1. Field of the Invention
The invention relates to brake control for controlling braking forces that are applied to wheels of a vehicle.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 2006-123889 (JP-A-2006-123889) describes a brake apparatus that achieves a target braking torque, which is determined based on an operation state and a traveling state of a vehicle, by performing regenerative braking control and hydraulic braking control in combination. Cooperative braking control is executed so that a total braking torque, which is a total sum of a regenerative braking torque and a hydraulic braking torque, matches a required braking torque that is a torque required by a driver. With this brake apparatus, it is possible to collect regenerative energy by executing regenerative braking control, thereby enhancing the fuel efficiency of the vehicle.
At the early stage of the cooperative braking control, a hydraulic pressure that should be achieved by the hydraulic braking control is temporarily raised by a large amount upon issuance of a command to execute the regenerative braking control. There is a time lag between when the command is issued and when a required regenerative braking force is actually achieved. Therefore, the hydraulic pressure is temporarily raised by a large amount in order to reliably obtain a required braking force during a period until the required regenerative braking force becomes available. After a sufficient hydraulic braking force is obtained, the hydraulic braking force is gradually decreased to gradually switch the braking control from the hydraulic braking control to the regenerative braking control.
However, in the hydraulic braking control, a predetermined-width dead-band region is usually set for a target hydraulic pressure in order to suppress occurrence of hunting in a hydraulic pressure. Therefore, even if the target hydraulic pressure is set to zero to switch the braking control from the hydraulic braking control to the regenerative braking control, it is determined that the target hydraulic pressure is achieved if an actual hydraulic pressure (i.e., wheel cylinder pressure) enters the dead-band region. This raises a possibility that the wheel cylinder pressure will not match zero and brake drag occurs. Such a problem may occur not only when the cooperative braking control is executed but also when pressure-decreasing control is executed, for example, when the target hydraulic pressure is set to zero.